1. Field of the Invention
The invention relates to cardiac pacemakers, and more particularly to a dual chamber rate responsive pacemaker which alters the pacemaker's V-A escape interval in response to the exercise level of the patient.
2. Description of the Prior Art
Various types of pacemakers are disclosed in the prior art.
The first pacemakers were asynchronous (VOO), and they stimulated the heart at a fixed rate, independent of the patient's underlying cardiac rhythm. Although such pacers, typified by U.S. Pat. No. 3,057,356 to Greatbatch, provide a ventricular pacing rate sufficient to sustain life, this pacing mode can compete with native ventricular rhythms which is undesirable.
Subsequently, demand pacemakers (VVI) were developed. This type of pacer interacts with the patient's heart to provide stimulating pulses only if spontaneous cardiac activity does not occur. An example of such a pacer is taught by U.S. Pat. No. 3,478,746 to Greatbatch. This form of pacer provides a ventricular sense amplifier for detecting ventricular depolarizations. A ventricular sense event resynchronizes the pacer's V-V timer by resetting and restarting it. The ventricular sense event also cancels or inhibits the scheduled ventricular stimulus and thus avoids competition with the native ventricular rhythm.
Also, at this time, atrial synchronized pacers (VAT) were developed. This type of pacer paces the ventricle in response to the detected atrial rate of the patient. The VAT pacer, as typified by U.S. Pat. No. 3,253,596 to Keller, provides an atrial sense amplifier for detecting atrial depolarizations. An atrial sense event starts the pacer's A-V delay timer. When the A-V timer times out, a ventricular stimulus is provided. Conceptually, such a pacer can be considered as a prosthetic conduction pathway which mimics the natural A-V conduction pathways of the heart. One drawback to this form of pacing is its ability to compete with ectopic ventricular activity. An ectopic ventricular beat (PVC) may be detected in the atrium which will result in the generation of a ventricular stimulus a short time after the ventricular depolarization. Although such a pacing regime is considered harmless if the A-V delay is short, it is possible to deliver the pacing stimulus into the vulnerable period of the ventricle and thereby initiate a lethal arrhythmia.
Continued development of pacemakers was marked by the invention of the A-V sequential pacer (DVI), as disclosed in U.S. Pat. No. 3,595,242 issued to Berkovits. This form of pacer provides for stimulation in both the atria and the ventricles while providing sensing in the ventricle. In this DVI mode pacer, a ventricular sense event starts both a V-A and a V-V timer. At the expiration of the V-A escape interval, the pacer delivers an atrial stimulus, and at the end of the V-V escape interval, the pacer delivers a ventricular stimulus. If a ventricular sense event occurs during the V-A or V-V time intervals, the pacer will resynchronize to the ventricular sense event and inhibit the delivery of the otherwise scheduled ventricular stimulus.
The DDI mode pacer described by U.S. Pat. No. 3,747,604 to Berkovits further includes an atrial sense amplifier to inhibit the atrial stimulus if an atrial sense event occurs during the V-A interval. The atrial sense event does not resynchronize the pacer which makes this device espcially suitable in patients where atrial competition must be avoided.
The atrial synchronized ventricular inhibited or VDD mode pacer, as disclosed in U.S. Pat. No. 3,648,707 issued to Greatbatch has structures for sensing in the atrium and ventricle but provides stimulating pulses only in the ventricle. In operation, the VDD pacer will synchronize on detected atrial activity and provide a ventricular stimulus if one does not occur within the A-V delay initiated by the atrial depolarization. Ventricular sense events both inhibit the delivery of the otherwise scheduled stimulus and also resynchronize the pacer's V-V timer.
More recently, the dual sense-dual pace DDD mode pacers, as first described by U.S. Pat. No. 4,312,355 to Funke, have been introduced. The DDD pacer addresses many of the shortcomings of the prior art devices.
The DDD mode pacer, as described by Funke, has had wide application. This type of pacer has sense amplifiers for detecting both atrial and ventricular events, as well as output pulse circuits for stimulating both the atrium and the ventricle. This form of prior art pacer provides timing circuitry to initiate an A-V delay upon the occurrence of an atrial event. If, during the A-V delay period, no spontaneous ventricular event is sensed, the pacer will produce a ventricular stimulus at the conclusion of the A-V delay period. Similarly, upon the occurrence of a ventricular event, the timing circuitry initiates a V-A delay period. If, during the V-A interval, no spontaneous atrial event is sensed, the pacer will produce an atrial stimulus at the conclusion of the V-A interval.
In this type of pacer, both the programmed A-V and V-A intervals are fixed, and in the absence of spontaneous P-waves or R-waves, the pacer will pace the heart at fixed A-A and R-R intervals. However, if the ventricle depolarizes spontaneously, then the A-V is truncated and the observed A-A interval is not fixed and will be shorter than the arithmetic sum of the programmed A-V and V-A intervals.
In contrast to this behavior, other forms of DDD pacing have been proposed and built. One such pacemaker which is important to the understanding of the present invention is described in U.S. Pat. No. 4,523,593 to J. C. Rueter. This pacer does not truncate the A-V delay period upon the occurrence of a ventricular sense event and the resultant pacer exhibits constant A-A paced intervals.
The dual chamber modalities, DVI, VAT, VDD and DDD, have proven to be especially efficacious pacemakers since they restore A-V synchrony and thus improve cardiac output by accommodating the hemodynamic contribution of the atrial chambers within the pacing regime. The latter three modes also synchronize the pacing rate to the patient's native atrial or sinus rate and thus provide an increased pacing rate in response to bodily activity. This rate responsive behavior is especially helpful for increasing cardiac output.
More recently, other pacers which increase cardiac output in response to exercise have been proposed. These include pacemakers which rely upon the sensing of a historical average of atrial activity, blood pH, respiratory rate or QT interval data to alter the pacemaker's escape interval. A discussion of these prior art proposals may be found in "The Exercise Responsive Cardiac Pacemaker", IEEE Transactions on Biomedical Engineering, Vol. 12, December 1984.
One approach which is important to the understanding of the present invention is the activity responsive pacer described in U.S. Pat. No. 4,428,378, issued to Anderson et al. The pacer disclosed in U.S. Pat. No. 4,428,378 monitors the physical activity of the patient and increases the pacing rate in response to increasing patient activity.
More recently, integration of activity sensing and dual chamber pacing has been proposed. See "Rate Responsive Dual Chamber Pacing" in PACE, Vol. 9, pp. 987-991.